1. Field of the Invention
The present invention relates to a process and tool for forming a compressed-radius hem on an outer panel to join it to an inner panel to form a sheet metal assembly.
2. Background Art
Hemming is a production process for joining an outer panel to an inner reinforcement panel. Conventional hemming processes are accomplished by bending a flange of the outer panel back onto the inner panel. Normally, a three-step process is used. In the first step, the outer panel is flanged with a perimeter portion being formed to extend substantially perpendicularly relative to the body of the outer panel. In the second step, the panel is pre-hemmed wherein the flange is formed to an acute angle of approximately 45xc2x0 to extend inwardly over a perimeter portion of the inner panel. In the third step, the panel is finally hemmed wherein the flange is formed to engage the inner panel and extend parallel to the body of the outer panel and perimeter portion of the inner panel.
Several different types of tools are used to perform hemming processes including reciprocating ram presses, tabletop hemming tools, and roll forming tools that may be manipulated by a robot. In conventional roll formed hem operations, a first pass is required to bend a 90xc2x0 flange to an intermediate angle of about 45xc2x0. The hem is closed in a second pass.
Recent developments in the field of hem forming have led to the development of reduced radius hems that improve the appearance of the fit of adjacent panels by reducing the perceived margin between adjacent panels. In conventional hem forming processes, the hem radius is controlled by the thickness of the inner and outer panels.
One problem with reduced radius hems is a tendency of the resulting hem to fracture near the tip of the hem if the part and tools are not properly aligned. One example of this hemming method and tooling is disclosed in U.S. Pat. No. 6,257,043 to Wiens and in Publication No. US 2001/0029766A1 that both relate to producing reduced radius hems. The final hem tool includes a flat section and an inclined section that can produce a hem with a reduced radius when compared to a conventional flat hem. In automotive manufacturing, for example, there are tolerances allowed both as to the relative location of the inner panel, outer panel, and the hem tool. These tolerances may result in variances with respect to the hem tool and flange location that are not a problem with conventional flat hemming techniques because the flat hem tool is not sensitive to inboard/outboard alignment of the tool relative to the flange. In the Wiens patent, if the final hem steel is too far inboard, the hem tip may be distorted and will either fail or result in an unattractive hem.
The use of a curved forming tool for forming a hem is disclosed in U.S. Pat. No. 6,000,118 to Biernat et al. that relates to a reciprocating ram press tool forming a sealed edge joint. The Biernat patent does not disclose a tool or method for producing a compressed-radius hem.
The disadvantages and shortcomings of the prior art are addressed by Applicant""s invention as summarized below.
According to one aspect of the present invention, a tool is provided for forming a compressed-radius hem on a sheet metal assembly comprising an inner panel having an outwardly extending flange and an outer panel having a bendable flange. The bendable flange is initially located generally perpendicular to an outer peripheral portion of the outer panel. The tool cooperates with a supporting surface on which the outer panel and inner panel are located. According to one embodiment of the invention, a roller having a cylindrical surface is used to bend the flange inwardly toward the surface of the outwardly extending flange of the inner panel. The roller also has a concave portion extending from a first circumferential line at the intersection of the concave portion and the cylindrical surface to a second circumferential line axially spaced from the first circumferential line. The concave portion is formed by at least two surfaces that together define a cavity relative to a chord extending between the first and second circumferential lines.
According to other aspects of the invention, the surfaces defining the cavity may include two or more partially conical surfaces that lie in two different coaxial conical sections that are coaxial with the cylindrical surface. Three, four, or more conical surfaces may be provided. The surfaces defining a cavity may also include a conical surface and a curved surface that is contiguous with the conical surface and also coaxial with the cylindrical surface. The curved surface may be located between the conical surface and the cylindrical surface and may be contiguous with both surfaces.
According to additional aspects of the invention, the cylindrical surface may be oriented to contact an inner portion of the flange as it is pressed against the outwardly extending flange of the inner panel while at least one of the surfaces forming the concave portion engages an intermediate portion of the flange that extends from a bight portion of the flange to the inner portion. A bend in the flange between the inner portion of the flange and the intermediate portion of the flange is preferably engaged by the roller near the first circumferential line. The concave portion preferably applies force in a direction normal to the intermediate portion of the flange. The cylindrical surface preferably applies force to the inner portion of the flange to flatten the flange against the outwardly extending flange of the inner panel.
According to yet another aspect of the invention, the same cylindrical surface used in the final hemming step may be used in the pre-hemming step to initially bend the flange from its initial generally perpendicular orientation to extend at an angle of about 45xc2x0 and partially over the outwardly extending flange of the inner panel. A concave portion in one embodiment may have an outer circumference that is greater than the cylindrical surface. In yet another embodiment, the concave portion may have an outer circumference that is less than the circumference of the cylindrical surface.
According to another aspect of the invention, a tool for forming a hem on a sheet metal assembly comprising an inner panel having an outwardly extending flange and an outer panel having a bendable flange comprises a support on which the outer panel and inner panel are located and a hem tool having a first surface that is parallel to the supporting surface, a second surface obliquely angled relative to the first surface and extending towards the supporting surface, and a third surface obliquely angled relative to the second surface and extending towards the supporting surface wherein the third surface is less oblique relative to the first surface than the second surface.
According to another aspect of the invention, the first surface is oriented to contact an inner portion of the flange as it is pressed against the outwardly extending flange of the inner panel while the second surface engages an intermediate portion of the flange that extends from a radiused portion to the inner portion. A bend in the flange between the inner portion of the flange and the intermediate portion of the flange may be engaged by the intersection of the first section and second section. The forming tool may be either a roller or a reciprocating ram press die.
According to another aspect of the invention, a method of hemming an outer metal panel having a perimeter flange and an inner metal panel together comprises the steps of placing the inner panel and outer panel together on a supporting surface. Forming a perimeter flange of the outer panel to extend generally perpendicularly relative to the body of the outer panel. In a pre-hemming pass, the perimeter flange is formed to an acute angle relative to the body of the outer panel with a roller. The perimeter flange of the outer panel is then formed in a final pass into engagement and over a perimeter portion of the inner panel. The roller has a first forming surface that is parallel to the perimeter portion of the inner panel and a second forming surface that is contiguous with the first forming surface and is oriented at a first oblique angle relative to the first forming surface. In the final hemming pass the roller may be oriented with its axis of rotation generally parallel to an intermediate portion of the perimeter flange. In this way, the bending force is focused on the intermediate portion of the flange so that the overlapping portions of the inner and outer panels are allowed to slide more easily in a parallel direction as the hem is formed in the final hemming pass.
The method may also include providing a third forming surface that is spaced from the first forming surface and contiguous with the second forming surface. The third forming surface may be oriented at a second oblique angle relative to the first forming surface that is less oblique than the first oblique angle.